The present invention relates to a borehole logging system and to a communication system for use in such logging systems. In particular, the invention concerns borehole logging systems which include a number of discrete logging sondes connected together to form an array, for example a borehole seismic array tool or to muliple measuring entities connected to form a string.
In the logging of boreholes, one method of making measurements underground comprises connecting one or more tools to a cable connected to a surface system. The tools are then lowered into the borehole by means of the cable and then drawn back to the surface (xe2x80x9cloggedxe2x80x9d) through the borehole while making measurements. The conductors of the cable provide power to the tool from the surface and provide a route for electric signals to be passed between the tool and the surface system. These signals are for example, tool control signals which pass from the surface system to the tool, and tool operation signals and data which pass from the tool to the surface system.
A schematic view of a prior art telemetry system for use in logging boreholes is shown in FIG. 1. The system shown comprises a digital telemetry module DTM which is typically located at the surface, a cable C, a downhole telemetry cartridge DTC at the head of a tool string which includes a number of downhole tools T1, T2, . . . each containing a respective interface package IP1, IP2, . . . through which they are in communication with the DTC via a fast tool bus FTB. This system is configured to handle data flows in opposite directions, i.e. from the tools, via the respective IPs and FTB, to the DTC and then to the DTM over the cable (xe2x80x9cuplinkxe2x80x9d), and the reverse direction from the DTM to the DTC and tools over the same path (xe2x80x9cdownlinkxe2x80x9d). Since the principal object of the system is to provide a communication path from the tools to the surface so that data acquired by the tools in use can be processed and analysed at the surface, the protocol used favours the uplink at the cost of the downlink to optimise data flow from the tools. The communication path is split into two parts, the cable C and the tool bus FTB, and operation of these two are asynchronous to each other. In the FTB, the uplink and downlink both comprise biphase modulation using a half duplex systems of identical instantaneous data rate and frequency synchronised to a clock in the DTC. Both uplink and downlink are half duplex. A suitable protocol for implementing such a system is described in U.S. Pat. No. 5,191,326 and U.S. Pat. No. 5,331,318, the contents of which are incorporated herein by reference. The FTB signal path is typically constituted by a pair of coax cables or a twisted pair conductor running along the length of the tool string.
The tools T1, T2 . . . in the tool string are typically a series of sondes which make physical measurements of the formation surrounding the borehole, for example electrical, nuclear and acoustic measurements. The sondes are usually connected together to form a rigid tool string with electrical connectors permitting data and power connection between or through the sondes. In use, the operator must configure the FTB from the surface system to indicate the number of nodes (i.e. number of tools or sondes) such that the system can allocate addresses for each node. Once this is set, it is fixed and must be completely reconfigured to change the number of nodes.
Certain borehole tools are commonly found in the form of arrays, in which a number of similar (or identical) sondes which make the same measurement are connected together. Such an approach is often found in borehole seismic logging tools and examples can be found in SEISMIC APPLICATIONS Vol. 1, CROSSWELL SEISMOLOGY and REVERSE VSP by Bob A. Hardage, Geophysical Press Ltd., London 1992. Because of the necessity to couple the measurement sondes closely to the borehole wall in such cases in order to improve the acoustic detection ability, and the difficulty of achieving such coupling with a very long tool string, it is often proposed to join the sondes together with lengths of flexible cable, often called xe2x80x9cbridlesxe2x80x9d. The Array Seismic Imager ASI tool of Schlumberger, the SST 500 tool of CGG and other examples of such xe2x80x9carrayxe2x80x9d or xe2x80x9cmulti-levelxe2x80x9d tools are found in U.S. Pat. No. 5,157,392.
One problem encountered with multi-level borehole seismic tools is that the large quantity of data recorded for each shot is greater than can be handled by current wireline telemetry systems. The tool described in U.S. Pat. No. 5,157,392 attempts to overcome this problem by providing memory in each sonde and in a downhole cartridge which is connected to the logging cable. In use, a signal is sent from a surface system to the cartridge to instruct activation of the measuring devices in each sonde for a predetermined time after the signal is received. This signal is coordinated with the firing of the surface source so that the sondes are active when the signal arrives. In order to overcome the limitations of the telemetry system, the sondes and the downhole cartridge are provided with buffers or memories which store the recorded signals. The stored signals are then telemetered to the surface over the logging cable when the sensors are not recording and when the tool is being moved in the borehole.
U.S. Pat. No. 5,585,556 describes a measurement while drilling system for making seismic measurements. In order to overcome the limitations of the telemetry system, signals are recorded downhole when drilling has stopped and a surface source is activated and stored. Some processing is performed on these signals and the processed data transmitted to the surface. The downhole tool must be retrieved in order to download all of the stored signals. In order to operate, the system is described as having synchronised clocks in the surface and downhole systems.
The systems described above have certain limitations. It is not possible to acquire data continuously and the surface system must be closely associated with the source firing system. This is often not possible, especially in marine environments. It is also not possible with this system to decide after the fact which data is to be telemetered to the surface and which can be discarded.
The present invention provides novel methods for recording data in borehole logging systems, novel borehole logging systems and novel borehole seismic logging tools and systems.
A method of recording data in a borehole logging system according to a first aspect of the invention comprises recording data at multiple measuring elements (such as seismic sensors) in a downhole system in a substantially continuous manner; storing the recorded data in a memory downhole; determining a data time window and a data sampling rate; and communicating, from the memory to the surface system, data falling in the determined time window and sampled at the determined sampling rate.
Preferably, time stamp data is associated with the recorded data in the memory. The time stamp data can be generated with a clock in the downhole system. In such a case, a synchronisation signal can be generated with a clock in the surface system, the synchronisation signal being sent to the downhole system and used to synchronise the clock in the downhole system with the clock in the surface system. The clock in the surface system can be synchronised with a time signal from a GPS system.
The time window and sampling rate can be communicated to the downhole system in a signal from the surface system. Alternatively, the time window and sampling rate can be determined in response to a detected event.
It is also convenient to transmit to the surface system data relating to the operating of the signal source which creates the signals sensed downhole.
The downhole system preferably includes a downhole telemetry cartridge and a sensor network cartridge, the recorded data being stored in the sensor network cartridge and the data being communicated to the surface via the downhole telemetry cartridge.
It is particularly preferred to assemble the downhole system at the surface and connecte it to the surface system and lower it into the borehole. By providing power to the downhole system, data can be recorded as the downhole system is lowered into the borehole.
A borehole logging system according to a second aspect of the invention comprises a surface system; and a downhole system, connected to the surface system, and including: a series of measuring elements; a memory; means for passing data from the measuring elements to the memory; and means for communicating data in a predetermined time window and at a predetermined sampling rate from the memory to the surface system.
A borehole seismic logging system according to a third aspect of the invention comprises a surface unit; a downhole seismic detector array connected to the surface unit and including a control module including a memory; and a series of shuttles, each of which has a sensor, the shuttles being connected to the control module and operating so as to record seismic signals and transmit data to the control module in a substantially continuous manner; wherein the control module communicates to the surface system data in a predetermined time window and at a predetermined sampling rate.
Preferably, the downhole system is connected to the surface system by means of a logging cable providing a power and data communication path.
The downhole array can further comprise a telemetry cartridge to which the control module is connected and via which it communicates with the surface system. Furthermore, the array can include a clock which provides time data to be associated with seismic signals recorded in the control module memory. The clock is preferably synchronised with a clock in the surface unit by means of control signals sent from the surface unit.
Where the system also includes a seismic source, the surface unit can receive time signals indicating operation of the source, the time signals being used to determine the time window and the sampling rate.
A borehole seismic logging tool according to a fourth aspect of the invention comprises a control module including a memory; and a series of shuttles, each of which includes a sensor and is connected to the control module such that, when supplied with power, it records seismic signal substantially continuously and transmits the recorded signals to the control module where they are recorded in the memory.
When the memory is full, it is preferred that new signals received from the shuttles are overwritten on old data already in the memory. The control module can also include a clock which provides time data to be associated with the recorded seismic signals. The control module preferably includes a first controller which can be connected to a surface system and a second controller which controls operation of the shuttles independently of any other borehole logging tools connected to the surface unit.